The present invention provides a novel method for recovering and reprocessing superabsorbent polymer fines. The method provides for the reconstitution of the fines back to superabsorbent polymers useful in applications, such as, diapers, sanitary napkins, soil additives, fuel filters, desiccants, cements, and sludge dewatering. The reconstitution method is similar to that set forth in co-pending U.S. patent application, Ser. No. 286,115, filed on Dec. 16, 1988,now U.S. Pat. No. 4,950,692, entitled "Method for Reconstituting Superabsorbent Polymer Fines". However, the novel process of the present invention provides for the simultaneous chemical modification of the superabsorbent polymer fines during the reconstitution process, thereby increasing the crosslinking in the superabsorbent polymer and, consequently, improving the gel strength in the resultant reconstituted superabsorbent polymer material.
Hydrogel-forming polymer compositions (hereinafter referred to as superabsorbent polymers) are used as absorbents in absorbent structures and absorbent articles. Synthetic superabsorbent polymers are typically produced by one of the following methods: gel, inverse suspension, or modified bulk polymerization. Typical gel methods for producing superabsorbent polymers are described in U.S. Pat. No. 4,654,039 (Brandt et al.) issued Mar. 31, 1987, and U.S. Pat. No. 4,286, 082 (Tsubakimoto et al.) issued Aug. 25, 1981. Typical inverse suspension processes for producing superabsorbent polymers are disclosed in U.S. Pat. No. 4,698,404 (Cramm et al.) issued Oct. 6, 1987, U.S. Pat. No. 4,446,261 (Yamasaki et al.) issued May 1, 1984, and U.S. Pat. No. 4,340,706 (Obayashi et al.) issued July 20, 1982. A modified bulk process is described in U.S. Pat. No. 4,703,067 (Mikita et al.) issued Oct. 27, 1987.
The Brandt et al. patent discloses a gel process comprising the steps of preparing a reaction mixture consisting essentially of particular amounts of unsaturated polymerizable acid group-containing monomers, crosslinking agent and optionally free radical initiator in an aqueous medium; subjecting this reaction mixture to polymerization conditions to produce a substantially water-insoluble, slightly crosslinked polymer material having under certain conditions particular gel volume, gel strength and extractable polymer content characteristics; and neutralizing at least a portion of the acid functional groups of the resulting polymer material with salt cations to form a partially neutralized polymer material having a degree of neutralization of at least 25%. The hydrogel material formed in accordance with the Brandt et al. patent may optionally be dried in order to prepare absorbent hydrogel-forming polymer materials which reform hydrogels upon subsequent contact with water or body fluids.
A typical inverse suspension process for producing superabsorbent polymers is disclosed in the Cramm et al. patent. This process provides for the suspending of an aqueous solution of acrylic acid and an alkali metal acrylate or ammonium acrylate, the mole ratio of the acrylic acid to the alkali metal acrylate or the ammonium acrylate being 50/50 to 2/98, in an alicyclic or aliphatic hydrocarbon solvent containing a surfactant having an HLB value of 8-12; subjecting the resulting suspension to inverse suspension polymerization in the presence of a water-soluble radical polymerization initiator and; if necessary, crosslinking the resulting polymer with a crosslinking agent, whereby the polymerization is conducted in the presence of a water-soluble chain transfer agent. The water-soluble chain transfer agent being formic acid.
Mikita et al. disclose a modified bulk process for producing superabsorbent polymers comprising the steps of combining potassium acrylate and a polyvinyl monomer with water in an amount of 55 to 80 combined weight percent of potassium acrylate and polyvinyl monomer based on the total weight of potassium acrylate, polyvinyl monomer and water to form a monomer mixture; and adding a polymerization initiator to the monomer mixture to initiate polymerization of the monomer mixture. The monomer mixture is polymerized without external heating by utilization of exothermic heat of reaction as substantially the only non-ambient energy source to drive water away from the polyacrylate resin to form the crosslinked polyacrylate resin having a water content sufficiently low, i.e., 15% or less, to be powdered, such as by pulverization, without an intermediate drying step.
Each of the aforementioned processes used to produce superabsorbent polymers may generate superabsorbent polymer fines. Superabsorbent polymer fines are defined as those particles which are too small in size for current market applications, e.g., diapers, sanitary napkins, etc. The fines may be generated either during the polymerization step of the process or during subsequent processing steps when the superabsorbent polymer is ground and sieved to meet desired particle size specifications.
As an example, in the production of superabsorbent polymer material for use as an absorbent in a certain personal product application all particles smaller than about +280 mesh are considered fines. Fines recovered from superabsorbent material used in this particular application may amount to between 5-35% of the total polymer produced.
Since superabsorbent fines are those particles which are too small in size for a specific market application, they are typically removed from a commercial product via a size classification step. The resulting fines are typically stored or disposed of until a suitable market can be found for them. Commercial applications of superabsorbent fines are limited by the small particle size which typically results in handling problems, such as, gel blocking and dusting.
Retaining fines in commercially used superabsorbent polymer material may also result in gel blocking, i.e., the fines block larger particles causing reduced absorbent capability of the total material. Due to gel blocking effects, fines are normally separated from the superabsorbent polymer material prior to commercial application.
Due to the high disposal, storage and handling costs associated with superabsorbent polymer fines, it is desirable to develop new methods for handling and utilizing these fines. One such process for recovering and reprocessing superabsorbent polymer fines is set forth in co-pending U.S. patent application, Ser. No. 286,115, filed on Dec. 16, 1988, now U.S. Pat. No. 4,950,692, and entitled "Method for Reconstituting Superabsorbent Polymer Fines", i.e., a method for reconstituting superabsorbent polymer fines which comprises the following steps: wetting the superabsorbent polymer fines sufficient to form an amorphous gel of the fines; grinding the gel; and drying the ground gel product
The present invention is an improvement on the aforementioned three step process for reconstituting superabsorbent polymer fines, whereby the polymer fines are simultaneously chemically modified during the reconstitution process so as to provide additional crosslinking throughout the superabsorbent polymer and, consequently, greatly improve the gel strength of the resultant reconstituted superabsorbent polymer material.
The ability of "hot" initiators such as sodium persulfate to crosslink polymers is well known. For example, the use of peroxide initiators, such as sodium persulfate, to crosslink the surface of polymers is disclosed in European patent No. 0 248 437 (Saotome), which was published on Dec. 9, 1987.
The Saotome patent discloses a process for improving a water absorbent polyacrylic acid polymer in which the polymer is contacted with an aqueous solution containing a water soluble peroxide radical initiator, followed by heating, thereby causing substantially only the surface portion of the polymer to undergo crosslinking with the radical initiator while leaving the remaining core portion of the polymer substantially intact. The weight ratio of the water of the aqueous solution to the polymer being from 0.01 to 0.5, thereby causing the polymer substantially only at its surface portion to have the solution absorbed therein. Subsequent to the contacting of the surface of the polymer with the aqueous solution containing a water soluble peroxide radical initiator, the mixture is heated at a temperature such that said initiator is decomposed while the polymer is not decomposed causing the polymer at its surface portion to undergo crosslinking by the action of the initiator while leaving intact the remaining core portion of the polymer. The surface crosslinking of the Saotome patent is achieved by using high solids, i.e., 66.7-99%.
The present inventors have discovered that addition of a free radical initiator into the aqueous solution used in wetting polymer fines during the reconstitution of such polymer fines results in the formation of a reconstituted polymer material having higher gel strength. Contrary to the Saotome patent, the present invention necessitates that the initiator be distributed uniformly throughout the polymer, not just at the surface. That is, the present inventors have discovered that the use of a lower solids levels, e.g., 5-50%, not only assists in the reconstitution of the polymer fines, but also ensures the uniform distribution of the aqueous initiator solution throughout the fines. By distributing the persulfate solution throughout the polymer fines, more uniform crosslinking (both internal and surface) is possible. Thereafter, the drying step generates initiator radicals which result in increased crosslinking throughout the polymer fines, thereby forming a reconstituted polymer material having higher gel strength.
Thus, the present invention provides a unique process which overcomes the disposal and handling problems associated with superabsorbent polymer fines, while forming a reconstituted polymer material which demonstrates higher gel strength verses conventional reconstituted polymer material.
Additional advantages of the present invention shall become apparent as described below.